The long-term goal of this project is to understand the mechanisms of eosinophilic inflammation in human diseases. In this current application, we will address a fundamental question, "what initiates the Th2-like immune response in eosinophil-associated disorders, such as bronchial asthma?" Specifically, we propose to study the roles and mechanisms of conventional NK cells (NK1.1+, CD3-) and eosinophils in the development of airway inflammation in bronchial asthma. Our focus on NK cells is based on striking and novel preliminary data demonstrating that mice deficient in NK cells, but with intact T cells, do not develop airway eosinophilia in response to an authentic fungal aeroallergen, Alternaria, which is implicated in the exacerbation of human asthma. Furthermore, human eosinophils do respond directly, without specific antibodies, to Alternaria by releasing proinflammatory mediators. These observations led us to hypothesize that the persistent and recurrent airway inflammation in bronchial asthma is mediated by dysregulated innate immune responses (i.e. NK cells during the initiation phase and eosinophils during the effector phase) to environmental factors. By using a mouse model, we will investigate the role of conventional NK cells (NK1.1+, CD3-) and the cytokines these cells produce in initiating eosinophilic airway inflammation in response to Alternaria (Aim 1a). The role of IL-5-producing NK cells in human asthma will be evaluated (Aim 1b). We will then investigate the effects of common environmental aeroallergens on the production and release of proinflammatory mediators by human eosinophils in vitro (Aim 2a) and study the pathological role of degranulating eosinophils in asthma using a novel mouse model (Aim 2b). These studies will discover and elucidate previously unidentified roles of innate immune responses in asthma and may profoundly change our understanding of the pathogenesis of diseases associated with tissue eosinophilia.